Portable handheld work apparatus such as chain saws, brushcutters, suction/blower apparatus or the like and equipped with an internal combustion engine as a drive motor include, as a rule, a membrane carburetor for forming an air/fuel mixture for the engine. In the membrane carburetor, a control chamber is provided, which is delimited by a membrane and is connected via a feed line to a fuel tank. A valve body is mounted in the feed line which clears the latter in response to a deflection of the membrane because of an underpressure in the control chamber. With an arrangement of this kind, fuel is continuously supplied in dependence upon the fuel quantity inducted via fuel nozzles in the carburetor and taken from the control chamber. At standstill of the engine, the induction underpressure is not present in the region of the fuel nozzles and therefore there is no flow of fuel through the nozzles into the intake channel. A pressure equalization in the control chamber accompanies standstill whereby the valve body is closed via the membrane and therefore an uncontrolled afterflow of fuel is avoided.
After a longer standstill of the carburetor, an at least partial emptying of the control chamber can occur because of vaporization processes or the like and this can lead to starting difficulties of the engine. The control chamber can be flooded with fuel by means of a suitable purge pump; however, undefined start conditions for the mixture formation can nonetheless occur.
It is an object of the invention to provide a membrane carburetor having improved starting characteristics for the internal combustion engine.
The membrane carburetor of the invention is for an internal combustion engine in a portable handheld work apparatus including a motor-driven chain saw, cutoff machine and brushcutter. The engine has a fuel tank and the membrane carburetor includes: a carburetor housing defining an intake channel communicating with the engine and through which a stream of combustion air flowing in an intake flow direction is drawn by suction when the engine is operating; the carburetor housing defining an interior space; a control membrane mounted in the interior space so as to define a control chamber therein bounded by the control membrane; a fuel supply line connecting the fuel tank to the control chamber; a control valve mounted in the fuel supply line and being actuable to control the inflow of fuel to the control chamber; the control membrane being operatively connected to the control valve for actuating the control valve to clear the fuel supply line in response to a deflection of the control membrane whereby fuel flows into the control chamber; at least one fuel nozzle; an ancillary chamber disposed in the carburetor housing; a fuel path connecting the control chamber to the fuel tank; a purge pump mounted in the fuel path; the control chamber being connected to the intake channel via the ancillary chamber and the fuel nozzle; the fuel path including a segment thereof between the control chamber and the purge pump; and, the fuel path being so configured that the segment thereof passes through the ancillary chamber.
With the above, a membrane carburetor is so configured that the fuel path between the control chamber and the purge pump is taken through at least one ancillary chamber communicating with the control chamber. In this way, it is ensured that vapor or air bubbles, which are possibly present in the ancillary chamber, can be purged with the purge pump. These vapor bubbles can lead, for example, to an uneven flow of the fuel through the fuel nozzle into the air channel of the carburetor and possible disadvantageous effects from the compressibility of such vapor bubbles are reliably avoided in this manner. The ancillary chamber can be in the form of a main nozzle chamber connected ahead of the main nozzle and, with an ancillary chamber having this form, fuel is made available in the direct proximity of the main nozzle via the purging operation so that fuel is available in the main nozzle at the beginning of the starting operation. A removal of fuel vapor, air or the like via suction through the main nozzle, which lengthens the starting operation, is thereby not present. With an arrangement of a check valve in the main nozzle chamber, a stickiness or gumming of the valve platelet, which is caused by a drying out, can be avoided or dissolved via a rinsing with fuel.
With an ancillary chamber in the form of a pump chamber of an accelerator pump, the pump chamber can be filled with fuel via a purging operation in advance of starting the engine. In this way, fuel is already available in the accelerator pump at the beginning of the starting operation. An engine, which runs unevenly in the starting phase, can be additionally supplied with fuel by actuating the throttle flap and the accelerator pump, which communicates with the throttle flap, in that fuel is injected into the air channel via a fuel nozzle. This achieves a short-term enrichment of the air/fuel mixture in the carburetor and facilitates a starting of the engine even under difficult climatic conditions. With an arrangement of several ancillary chambers (especially the primary nozzle chamber and the pump chamber), these chambers are advantageously flow-conductingly connected in series to the purge pump. With a purging operation, the ancillary chambers are sequentially purged and the discharge of vapor bubbles or air bubbles is ensured. With a flow-conducting parallel connection of two or more ancillary chambers or even one ancillary chamber to the control chamber, these chambers can be effectively thoroughly purged while avoiding an interaction with each other.
In a practical embodiment, the purge pump is configured as a suction pump acting on the control chamber and is mounted in a return line to the fuel tank. A purging operation is made possible especially in combination with check valves mounted in the region of the ancillary chambers and operating on the corresponding fuel nozzle with this purging operation reliably eliminating vapor bubbles and avoiding a premature discharge of fuel via the fuel nozzles. The check valves prevent an unwanted induction of air via the fuel nozzles.
In an advantageous further embodiment, the valve body, which is actuated by the membrane, is mounted laterally in the membrane carburetor. The fuel path connects the control chamber to the fuel tank and a segment of the fuel path is between the control chamber and the purge pump. On the same side of the carburetor, a portion of this segment of the fuel path is provided, which connects the ancillary chamber to the purge pump whereby, overall, a space-saving configuration is made possible and conventional carburetors can be replaced with an embodiment according to the invention while retaining the external measurements.
The purge pump advantageously has an elastically depressible pump bellows, which is preferably transparent and via which the control chamber with the connected ancillary chambers can be thoroughly purged with a thumb or a finger. The elastic pump bellows is at least approximately free of wear and permits the user to reliably determine whether bubble-containing fuel is pumped during the purging operation. The purging of pure fuel can be reliably seen whereby a conclusion can be drawn as to an adequate purging of the control chamber and the connected ancillary chambers.
The purge pump advantageously has a valve arrangement with at least one duckbill valve and advantageously includes a suction valve and a pump valve which operate alternately. The pressure valve, which is configured as a duckbill valve, seals the fuel line with two elastic mutually adjacent sealing lips even when there is no external counterpressure. A high resistance to wear is provided by the omission of mechanically movable valve bodies. The induction valve for inducting fuel from the control chamber is advantageously configured as a valve having a valve plate. Especially in combination with the elastic depressible pump bellows and its relatively low suction forces, the large surface of the valve plate leads to an opening of the suction valve even for a slight pressure difference and therefore leads to a high throughflow rate during induction.